As known in the prior art, a principal object of heat recovery systems is to replace primary energy in an economical way. In heat recovery systems, heat exchangers are used, of which the most important types are a plate heat exchanger and a tubular heat exchanger. In prior art plate heat exchangers, a plate structure forms two systems of ducts perpendicular to one another, and a medium that delivers heat flows in one set of ducts and a medium that receives heat flows in the other set of ducts, the heated receiving medium being passed further to reuse. Tubular heat exchangers are generally provided with a supply of steam or water, and the tubes are surrounded by ribs or equivalent so as to increase the heat exchange area. In lamellar radiators, the tubes are typically fitted between a plate structure, and water flow in the ducts formed by the plate structure, for example glycol water.